246 NATURAL HISTORY OF AQUATIC INSECTS CH. | 



careful to select such as have a suitable specific i 



gravity. | 



" Tiie larvae cannot swim, or can only swim badly, ! 



and they usually creep about. In the act of creeping, { 



the head and fore-part of the body are thrust out of j 



the sheath, and the larva drags itself along by its six | 



legs. This operation is often greatly facilitated by | 



the buoyancy of the vegetable fragments attached to | 



the sheath. [Some young larvae can swim very j 

 fairly. In spring and early summer a small Caddis- 

 worm, which is common in ponds, may be seen 

 to swim rather rapidly through the water with the 



help of its long, fringed hind-legs. The movement ij 



is a little jerky, but the larva, carrying its case, can ;j 



move through the water in any direction in a straight ,) 



line.] :• 



" Caddis-worms have six hard and jointed legs. ■ 



They have no false abdominal feet, like those of ;i 



caterpillars. The head is protected by armour of a J! 



brown colour. The first ring of the thorax is of the il 



same texture and colour, and bears a short pair of \ 



legs. The second ring is also brown and hard, and » 



carries the second pair of legs ; the third ring is i 



yellowish with brown spots, and carries the third ^ 



pair. The rest of the body is made up of nine rings, j 



which are white, soft and transparent. On the first I 



of these nine abdominal rings are three fleshy / 



prominences more or less conspicuous in the different ij 



species. One of these projects from the upper surface '? 



of the ring, while the other two are lateral ; all can be d 



retracted at pleasure. They are hollow at the tip, )| 



and when the Insect is taken out of the water, they ■! 



